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Trace the network connection close sequence on the Database Engine

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This article presents examples of a network trace that captures the sequence during when a Transmission Control Protocol (TCP) connection between a client application and the SQL Server Database Engine (the server) is closed. Understanding these patterns is crucial for diagnosing network behavior, identifying pooling strategies, and optimizing connection management in web or service applications.

Closing connection types

This article provides examples for normal TCP connections, and Multiple Active Result Sets (MARS) connections. MARS is a feature of SQL Server, introduced with SQL Server 2005 (9.x), that allows multiple commands to be executed on a connection without having to clean up the results from the first command, before running the second command. MARS is achieved through session multiplexing (SMUX).

This section describes several examples of closing a network connection.

  • The client IP address is 10.10.10.104
  • The server IP address is 10.10.10.22

Closing packets

This example shows a normal connection close sequence. Note the low frame numbers and time offsets. This sequence is most likely a pooled connection closure. This should occur within 30 seconds of the beginning of the trace, or you might also see keep-alive packets.

Diagram of four-way TCP session close. 

Frame Offset    Source IP    Dest IP      Description
----- --------- ------------ ------------ ---------------------------------------------------------------------------
   50 4.1529661 10.10.10.104  10.10.10.22 TCP:Flags=...A...F, SrcPort=4657, DstPort=1433, PayloadLen=0, Seq=413460761
   51 4.1529661  10.10.10.22 10.10.10.104 TCP:Flags=...A...., SrcPort=1433, DstPort=4657, PayloadLen=0, Seq=280398321
   52 4.1529661  10.10.10.22 10.10.10.104 TCP:Flags=...A...F, SrcPort=1433, DstPort=4657, PayloadLen=0, Seq=280398321
   54 4.2330441 10.10.10.104  10.10.10.22 TCP:Flags=...A...., SrcPort=4657, DstPort=1433, PayloadLen=0, Seq=413460761

Transact-SQL statements and closing packets

This example shows closing a nonpooled connection, after two Transact-SQL statements. If this connection was nonpooled, you could also see keep-alive packets associated with sending the connection back into the connection pool, instead of the closing packets immediately following the last response from the server. We recommend pooling connections in any sort of web or service application to allow connection reuse. Connection pooling reduces the number of connections to the server, and minimizes the cost and delay associate with new connections.

Frame Offset    Source IP    Dest IP      Description
----- --------- ------------ ------------ ---------------------------------------------------------------------------
  364 9.1949581 10.10.10.104  10.10.10.22 TDS:SQLBatch, Version = 7.300000, SPID = 0, PacketID = 1, Flags=...AP..., S
  365 9.1949581  10.10.10.22 10.10.10.104 TDS:Response, Version = 7.300000, SPID = 130, PacketID = 1, Flags=...AP...,
  366 9.3043331 10.10.10.104  10.10.10.22 TDS:SQLBatch, Version = 7.300000, SPID = 0, PacketID = 1, Flags=...AP..., S
  367 9.3072631  10.10.10.22 10.10.10.104 TDS:Response, Version = 7.300000, SPID = 130, PacketID = 1, Flags=...AP...,
  375 9.4078491 10.10.10.104  10.10.10.22 TCP:Flags=...A...F, SrcPort=4647, DstPort=1433, PayloadLen=0, Seq=157672648
  376 9.4078491  10.10.10.22 10.10.10.104 TCP:Flags=...A...., SrcPort=1433, DstPort=4647, PayloadLen=0, Seq=192890973
  379 9.4078491  10.10.10.22 10.10.10.104 TCP:Flags=...A...F, SrcPort=1433, DstPort=4647, PayloadLen=0, Seq=192890973
  397 9.5221071 10.10.10.104  10.10.10.22 TCP:Flags=...A...., SrcPort=4647, DstPort=1433, PayloadLen=0, Seq=157672649

Idle or pooled connection being closed

The connection is closed 10 seconds after the previous keep-alive exchange (see Delta column).

Note

The parser mistakenly marks the initial ACK+FIN packet (Frame 1881) as a keep-alive ACK packet, because the previous keep-alive packet. However, it is initializing the connection closure.

Frame Offset     Delta      Source IP   Dest IP     Description
----- ---------- ---------- ----------- ----------- -----------------------------------------------------------------
 1314 16.3641802  0.0000000 10.10.10.45 10.10.10.51 TCP:[Keep alive]Flags=...A...., SrcPort=51708, DstPort=1433, Payl
 1317 16.3677083  0.0035281 10.10.10.51 10.10.10.45 TCP:[Keep alive ack]Flags=...A...., SrcPort=1433, DstPort=51708, 
 1327 16.4269375  0.0592292 10.10.10.51 10.10.10.45 TCP:[Keep alive]Flags=...A...., SrcPort=1433, DstPort=51708, Payl
 1328 16.4269637  0.0000262 10.10.10.45 10.10.10.51 TCP:[Keep alive ack]Flags=...A...., SrcPort=51708, DstPort=1433, 
 1881 26.7918499 10.3648862 10.10.10.45 10.10.10.51 TCP:[Keep alive ack]Flags=...A...F, SrcPort=51708, DstPort=1433, 
 1886 26.7929474  0.0010975 10.10.10.51 10.10.10.45 TCP:Flags=...A...., SrcPort=1433, DstPort=51708, PayloadLen=0, Se
 1888 26.7929474  0.0000000 10.10.10.51 10.10.10.45 TCP:Flags=...A...F, SrcPort=1433, DstPort=51708, PayloadLen=0, Se
 1890 26.7929947  0.0000473 10.10.10.45 10.10.10.51 TCP:Flags=...A...., SrcPort=51708, DstPort=1433, PayloadLen=0, Se

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